Dana 20 Transfer Case — Specs, Service, and Limitations

Difficulty 2/52–6 hrs$80–3501966-1977

The Dana 20 is a robust, gear-driven transfer case with a 2.03:1 low range — adequate for mild trail use, marginal for serious rock crawling. Service it every 30,000 miles and watch the output shaft seals.

All 1966–1977 Broncos used the Dana 20 part-time transfer case. It's a chain-free, gear-driven unit — extremely reliable and straightforward to service. Key specs:

Low range ratio: 2.03:1
High range: 1:1 (direct)
Shift: floor-mounted twin levers (2H / 4H / 4L)
Input: from the transmission output
Front output: to the front driveshaft

**Crawl ratio calculation:** With a 3-speed manual + 3.07:1 first gear + 2.03:1 low range + 4.09:1 axle ratio: 3.07 × 2.03 × 4.09 = **25.5:1 crawl ratio.** Acceptable for mild trails; inadequate for rock crawling without steep axle gearing.

**Service schedule:**

Drain and refill gear oil every 30,000 miles. Use 80W-90 GL-5 gear oil. Capacity: ~1.5 pints.
Inspect output shaft seals annually. Leaking rear seal soaks the driveshaft yoke and eventually destroys the driveshaft U-joint. A $18 seal prevents a $250 U-joint.
Inspect the front output yoke for elongation — the nut loosens over time on high-mileage units.

**Shift problems:** If the Dana 20 won't shift into 4L or pops out of low range, the most common cause is a worn detent ball and spring in the shift rail. A rebuild kit ($180) includes these along with all seals.

**Upgrade path:** Some builders swap to a Dana 300 (1980–1986 CJ) for a 2.62:1 low range. This is a custom fabrication project — the bolt pattern and dimensions differ. Not a common early Bronco modification.

Why it works

gear-driven (no chain to stretch or replace)
straightforward and proven design
easily rebuilt
cheap parts

Trade-offs

2.03:1 low range is modest vs. modern transfer cases
limited crawl ratio without steep axle gearing
twin-lever shift can be notchy when cold

Tools required

drain pan
socket set
torque wrench
gasket scraper

Parts

Part	Vendor	Est. price
Dana 20 rebuild kit (bearings, seals, gaskets)	Spicer / Various	~$180
Dana 20 output shaft seal (rear)	National Seals	~$18
Gear oil 80W-90 (1 quart)	Pennzoil / Mobil 1	~$12

Sources

{'title': 'Dana 20 service information', 'url': 'https://www.jeep4x4center.com/dana20.htm'}
{'title': 'Transfer case identification guide', 'url': 'https://www.broncodriver.com/'}

Written and maintained by an AZ wheeler and driveway wrencher. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.